Coaxial rotorcraft have been known for many years. However, because of difficulties involved in the control of cyclic and collective pitch of rotor blades in a coaxial configuration, development of this type of aircraft has heretofore been limited. Conventional coaxial designs provide roll, pitch and yaw control by providing control input linkages for cyclic and collective pitch to both an upper rotor and a lower rotor of a coaxial rotor set. This has conventionally involved providing at least two swash plates. One below, and one above, the lower rotor, to transfer control inputs past the lower rotor to the upper rotor, which is rotating in the opposite direction.
Several successful coaxial designs have been developed, for example, those by Nikolai Kamov and the Kamov Design Bureau of the former Soviet Union. The Kamov organization continues to produce coaxial rotorcraft in the Russian Federation. Other coaxial designs exist, for example a small coaxial unpiloted craft developed by United Technologies Corporation of Hartford, Conn. An example of the control system for this latter craft is disclosed in U.S. Pat. No. 5,058,824.
Coaxial designs are advantageous because they eliminate the need for a tail rotor, and are generally more efficient. With a coaxial design, one way of providing yaw control is to provide a differential collective blade pitch control. Pitch is increased in one rotor, and decreased in the other, to unbalance torque. Another way of providing yaw control is to place one or more airfoils in the rotor set downwash. The airfoils are tiltable with respect to the downwash. The airfoils, nominally set to provide minimal air resistance in the downwash, intercept and redirect the downwash from the rotor set by tilting in one direction or the other from this initial position. This creates a reaction force vector at a location away from a yaw axis of rotation of the airframe and tends to yaw the airframe right or left depending on which way the airfoils are tilted. An example of such a system is disclosed in U.S. Pat. No. 5,791,592, issued Aug. 11, 1998 to Nolan, et al. In the Nolan system, there is no cyclic blade pitch control, as pitch and roll control are provided by tilting the rotor set with respect to the airframe; thus, the thrust vector from the rotor set is deflected with respect to the airframe to pitch and roll the aircraft.
It has been recognized that instead of a differential collective blade pitch control where the blade pitch of one rotor is increased as that of the other is decreased, and vice versa, by the same amount, that collective blade pitch control inputs of different amounts can be made to the respective rotors of the rotor set. In one example, a single collective blade pitch control of one rotor only can provide a yaw attitude control input for the coaxial rotorcraft a coaxial rotorcraft. Commensurate potential advantages of performance achievable for potentially lower cost also argue for simplification in design.
In another example, as described in U.S. Appl. Pub. No. 2006/0102777, a control system for a rotorcraft having a coaxial rotor set including a first rotor carried by a first drive shaft, and a second counter-rotating rotor carried by a second drive shaft, has a collective pitch control which provides a yaw attitude control input via a collective control input to one rotor, without providing a collective control input to the other rotor. The rotorcraft can have another collective control system which is essentially independent of the yaw control collective, providing a separate control input for rotor thrust and a separate control input for yaw. This can still be simpler than providing a differential collective where one increases while the other decreases by the same amount. Common to these is that a yaw attitude control input is provided by enabling a different blade pitch magnitude for the first rotor as compared to the second rotor, thereby unbalancing the torque forces in the coaxial rotor set.
One implementation of this prior art control system is to provide collective control on only one rotor of the set, either the upper rotor or the lower rotor. A cyclic pitch control can be provided to enable pitch and roll attitude control inputs. This cyclic pitch control can also be limited to one rotor of the rotor set in one implementation; and in one implementation it can be the same rotor to which a collective control is applied.
In other implementations of this prior art control system, yaw control can be supplemented by a tail rotor. Such a tail rotor does not draw power constantly, but only for brief periods of time in order to provide yaw control. For at least this reason, the tail rotor can be small, and can comprise a ducted fan. Moreover, in further detail, variations can include replacing the tail rotor with an air jet and providing yaw paddles to supplement yaw control and to provide directional stability.
These prior art control systems rely on aerodynamic effects to provide lift, yaw, pitch, and roll control of the rotorcraft. Exemplary embodiments of the present invention include inertial control approaches that may greatly simplify construction and control algorithms of rotorcraft.